The long term aim of this work is to understand the functional and structural properties of an inactivating form of a large-conductance voltage-and calcium (Ca 2+)-activated potassium (K+) channel (termed BK channel). BK channels are a widely distributed family of ion channel proteins found in neurons, skeletal muscle, smooth muscle, endocrine cells, and other tissues. The physiological roles of BK channels remain unclear, although BK channels are typically thought to act as passive sensors of membrane voltage and submembrane Ca2+. In some cases, BK channels may play a role in rapid repolarization after action potentials. In contrast to most previously described BK channels, which exhibit sustained activation during depolarization or elevation of Ca2+ (BKs channels). BK channels found in rat chromaffin cells and the PC12 clonal cell line exhibit rapid inactivation (BKi channels). This inactivation shares a number of features with an N-terminal inactivation exhibited by members of the voltage-dependent K+ channel family suggesting that the mechanism of inactivation may be similar. Using single channel and whole-cell electrophysiological methods, this project will address three issues about the functional properties of BKi channels. First, the mechanism of inactivation of BKi will be determined and compared to inactivation of other voltage-dependent channels. Second, the Ca- and voltage-dependence of activation of BKi channels will be compared to BKs channels. Third, using cells which express either BKi or BKs channels, the physiological roles of BKi and BKs channels will be compared. Because of the widespread distribution of BK channels among cells, the potential importance of this ion channel to normal cellular physiology and health is apparent. This proposal will contribute significantly to our understanding of the functional properties and physiological role of an important class of K+ channel.